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Proper Tire Air Pressure; Tell me if you've heard this one...
Topic Started: Jul 17 2015, 08:33 PM (1,923 Views)
Bad Bent
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Facetious Educated Donkey

OK, for the most part we are Fuel Efficient Freaks and like to max out our tire pressures. That is: max. XXXX pounds at XX psi (XX BAR). I read 35 psi on my 12" tires and add two for fun. :banana

The story:

So I work for THE hospital in town and we take our company cars to Bruneel Tire, a block from the hospital. I noticed some outer wear on the right front and asked if they would check the tire pressures.

I said 'I'm not sure if it's tire pressure or what. They are rated at 44 psi.'

Young man drags the hose over, pops the hub cap, then proceeds to go to the driver's door to look for the recommended tire pressure. Does not find a sticker.

:deal He says: 'The 44 psi is for the maximum pressure. The door sticker would give what the pressure is for the car to drive and steer properly, usually about 32 pounds. If the tire temperature goes up one degree it adds X pounds*, so we fill to the sticker pressure.'

*I do not remember clearly but I think he said 1 degree rise in temp. adds 10 pounds of pressure, could have been 2. :dunno I didn't challenge him on anything, it being the company car etc.

When I filled the car with fuel, I checked and all the tires were showing some outer and inner wear, perfect center tread, so I bumped their pressures up to 40 psi with my own gauge; from an average 32 psi (warm tires).

FYI, my 1991 Geo door sticker says 32 psi and the tires are rated at 35. I assume the door sticker is a engineered safety issue.

Insurance? CYA? Conspiracy? All of the above. Your take on this, please. :)
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Johnny Mullet
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Fear the Mullet

As long as they are not flat, keep rolling :rad
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myredvert
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myredvert

Quote:
 
FYI, my 1991 Geo door sticker says 32 psi and the tires are rated at 35. I assume the door sticker is a engineered safety issue.

Insurance? CYA? Conspiracy? All of the above. Your take on this, please. :)
The general rule is 1 psi for every 10 degrees temperature change.

I doubt it's a "safety/CYA" issue since the maximum rated pressure already has industry standard safety margins considered. No real purpose or need to further decrease pressure for that reason, especially considering many performance aspects may be degraded at lower pressures.

Door sticker pressures are going to a compromise the designers settled on considering handling, braking, wear, and ride.

The more computerized systems become, the more significant the recommended pressure becomes, because performance and certification testing would have been done at that pressure, and we don't know what accommodations were made in the programming to get the best performance obtainable at the "sticker pressure" that may result in degraded performance if the pressure is changed from "spec."
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Old Man


I am NOT airing up the damn CAR. Im airing up the TIRE. Air THE TIRE up to max stated on the TIRE or you don't get PAID...............
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Woodie
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The number on the tire is the maximum safe pressure for that tire, without regard to what car it is installed on. The car manufacturer recommends the pressure on the B-pillar sticker based upon as myrevert already said "a compromise the designers settled on considering handling, braking, wear, and ride". Any compromise can be skewed another direction by someone who's agenda is different, that's why there is NO SUCH THING AS PROPER AIR PRESSURE. Just as there is no such thing as the correct beer, thousands of choices and no one is right or wrong. If you think gas mileage is the most important thing, then the max pressure on the sidewall is the proper choice. You'll have less rubber on the road and a harsher ride, but you'll get better mileage and probably longer tire life. If you want to be more like Ford during the Explorer/Firestone fiasco and believe that making a truck ride cushy like a car is the most important thing in the world, then put in less than the tire manufacturer says is safe and just kill your customers, one family at a time. Then, blame it on the tire manufacturer who has a folder full of written warnings where they told you repeatedly that it was unsafe.
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myredvert
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myredvert

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I am NOT airing up the damn CAR. Im airing up the TIRE. Air THE TIRE up to max stated on the TIRE or you don't get PAID...............
:rofl
Big waste of time too. Last time I increased the pressure in my car, I spent a week sealing everything up well enough to do it, it took 2 days just to get the cabin pressure up to 32 psi, then as soon as I opened the door it went right back down to ambient... :smackface
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Car Nut
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myredvert
Jul 18 2015, 07:21 AM
Quote:
 
I am NOT airing up the damn CAR. Im airing up the TIRE. Air THE TIRE up to max stated on the TIRE or you don't get PAID...............
:rofl
Big waste of time too. Last time I increased the pressure in my car, I spent a week sealing everything up well enough to do it, it took 2 days just to get the cabin pressure up to 32 psi, then as soon as I opened the door it went right back down to ambient... :smackface
:rofl
Good one there. Almost spit my coffee on the screen.
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rjsdotorg
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http://web.archive.org/web/20060213020513/http://www.officer.com/article/article.jsp?siteSection=19&id=27281
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Bad Bent
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Facetious Educated Donkey

rjsdotorg
Jul 19 2015, 07:54 PM
Good one. :thumb :thumb
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Cardinal Grammeter
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I have strong feelings about tire pressures that obtain even wear.

The absolute best way to do it, PERIOD, is with tire temperature.

You measure it in 3 places, inboard, center, and outboard.

For pressure, set the center to the average of the in and outboards.

You will get the best grip, too, going around corners - it can be dramatic.

You can go a step further and look and the in and out temps.

For you given camber settings, you can adjust the toe-in to balance in-out temps. Then you can get uniform temp across the entire tire.

When you do this, grip increases even more.
_______________________

Now it gets even more complicated.

If you AutoX the car with this setup, you check for tires running on their sidewalls in the corners.

When this happens, you HAVE to increase pressure to get the wear back on the tread.

But that blows out the temp profile.

So how to fix:

More caster and camber.

When you get c&c right, then you can run an even temp AND keep the wear on the tread.

...and get more grip.

But what does this mean?

Now the alignment and pressure have been optimized for AutoX - and NOT FOR INTERSTATE TRIPS!!!

So depending on how you drive your car, there is no one, proper alignment/pressures.

But you can tune it just the way you want WITH TEMPS!

You don't need alignment racks, lasers, plumb bobs, squares, lines, etc.

But there is one more level of tuning and that is how the car behaves transiently - how it turns in, etc.

That, is beyond my level of understanding. But I can tell you rear toe in is a big deal. I'll leave that to someone else to explain which I would LOVE to hear!

That's all I got :o)
Edited by Cardinal Grammeter, Aug 7 2015, 05:00 PM.
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dayle1960
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Fastest Hampster EVER

I have an uncle who worked in a Uniroyal plant in Alabama. I once asked him about pumping the psi in my metro tires up to 50 psi and if I did so would it harm the tires. He asked how heavy my metro was and I told him about 1600#. He pondered my query for about a second and then said it would not hurt a thing. Only it would lead to the center of the tire wearing a bit faster than the outer edges. Since I've owned my metro (for about 15 years now) I've always aired the tires up to 50 psi. Winter or summer, it does not matter. I don't feel it is any unsafer than 32 psi. Once you get used to the hardness of the ride, I feel the better mpg's overshadow the more pleasant ride of 32 psi.

I passed by a gravel dump truck the other day and looked at the sidewall. The air pressure recommendation was 150 psi. Now with a truck which could haul TONS of gravel, I wonder why its air pressure is only 100 #'s more than my metro. Watching the Tour de France, a commentator stated that some contestants aired up those thin tires up to 50 psi. Now these two facts got me wondering if our metros tires could be aired up to 100+ psi with no problems. Seems probable, huh?

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rjsdotorg
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dayle1960
Aug 7 2015, 06:00 PM
Watching the Tour de France, a commentator stated that some contestants aired up those thin tires up to 50 psi. Now these two facts got me wondering if our metros tires could be aired up to 100+ psi with no problems. Seems probable, huh?

50?
My road clinchers are sidewall of 110psi, and a lot of tubular tires (pros) run 150.
Posted Image

I heard once that the sidewall max is ~50% of a tire's average explosion pressure.
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Physics_Outlaw
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Physics_Outlaw
[ *  *  * ]
Quote:
 
Now these two facts got me wondering if our metros tires could be aired up to 100+ psi with no problems. Seems probable, huh?


I wouldn't want to be anywhere near a tire rated for 35psi being filled to 100psi. If the tire decides to come apart, it will be violent. Safety first. :die
Edited by Physics_Outlaw, Aug 7 2015, 07:50 PM.
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RONNIEREDLINE
METRO MAD MAN

if you have the most sensitive pressure gauge in the world and take the tire pressure with the valve at 6 o clock then due it with the valve at 12 o clock it should be a smiggin less at 12 o clock do to atmospheric pressures.
true but RREEEAAAALLLLYYYYY stretching it. this was on an engineers test as a trick question but it is real.
redline
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RONNIEREDLINE
METRO MAD MAN

any vehicle only ways what the tire pressure is per square inch.
so if you vehicle has 32 psi in the tire then it is only putting out 32 pounds of gravitational pull per square inch of the tire contact pressure. if the truck has 150 psi he can only weight 150 pounds per square inch of his tire contact on the road.
if his load is more than the tire can handle then the tire will bulge to make a bigger contact area and add more square inches per area on contact.
tires dont move they rotate , actually no matter how fast you go, a tire isnt moving when it is in contact with the road it is rotating.
another engineers trick question on a test.
hmmmm but true
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